Abstract

Sediment hydraulic properties and consolidation behavior are key parameters that affect pore pressure generation, fluid migration, deformation, and slip behavior and mechanical strength of subduction zone megathrusts. We report on a set of constant rate of strain consolidation experiments and isostatic deformation and permeability experiments on whole-round core samples obtained during Integrated Ocean Drilling Program (IODP) Expedition 343 as part of the Japan Trench Fast Drilling Project (JFAST). The samples were collected from the frontal prism at 697.18 meters below seafloor (mbsf) and the underthrust sediment section at 831.45 mbsf at IODP Site C0019, 6.0 km landward of the Japan Trench. The permeability of sediment from the frontal prism decreases from 7.3 × 10–18 to 3.5 × 10–19 m2 as effective mean stress increases from 14.6 to 62.3 MPa under a uniaxial loading path and from 5.4 × 10–18 to 2.9 × 10–19 m2 over effective stresses from 3.6 to 54.6 MPa under an isostatic loading path. Porosity decreases from 46% to 27% as effective mean stress increases from 0.73 to 62.3 MPa and 46% to 29% as stress increases from 3.6 to 64.6 MPa for uniaxial and isostatic loading of the prism sediment, respectively. Permeability of the underthrust sediment sample decreases from 4.0 × 10–18 to 3.2 × 10–19 m2 as stress increases from 3.6 to 86.6 MPa under isostatic conditions. Porosity decreases from 49% to 37% over the stress range from 0.73 to 69.6 MPa and 48% to 36% from 3.6 to 86.6 MPa for uniaxial and isostatic loading paths, respectively. For both samples, permeability exhibits a log-linear decrease with decreasing porosity. In situ permeability for the prism and underthrust sediment samples are estimated from our laboratory-defined permeability-porosity relationships. The in situ porosity values for the prism sediment samples are 44% and 45% for uniaxial and isostatic conditions, respectively. The corresponding in situ permeability values are 4.3 × 10–18 and 4.9 × 10–18 m2. In situ porosity for the underthrust sample is 47% under both uniaxial and isostatic conditions. The permeability value for the underthrust sediment sample is 4.9 × 10–18 m2 under isostatic loading conditions. The coefficient of consolidation (Cv) decreases with increasing effective axial stress to near-constant values of 4.2 × 10–7 and 9.0 × 10–6 m2/s, and the compression index (Cc) is 0.70 and 0.57 for the prism and underthrust sediment samples, respectively. Preconsolidation stresses (Pc′) for the prism and underthrust samples, defined using the Casagrande method, are 17.0 and 20.5 MPa, respectively, and correspond to overconsolidation ratios of 3.1 and 2.8.